The heart of a computer is an assembly that is referred to as a magnetic disk drive. The magnetic disk drive includes a rotating magnetic disk, write and read heads that are suspended by a suspension arm adjacent to a surface of the rotating magnetic disk and an actuator that swings the suspension arm to place the read and write heads over selected circular tracks on the rotating disk. The read and write heads are directly located on a slider that has an air bearing surface (ABS). The suspension arm biases the slider into contact with the surface of the disk when the disk is not rotating but, when the disk rotates, air is swirled by the rotating disk. When the slider rides on the air bearing, the write and read heads are employed for writing magnetic impressions to and reading magnetic impressions from the rotating disk. The read and write heads are connected to processing circuitry that operates according to a computer program to implement the writing and reading functions.
The write head includes a coil layer embedded in first, second and third insulation layers (insulation stack), the insulation stack being sandwiched between first and second pole piece layers. A gap is formed between the first and second pole piece layers by a gap layer at an air bearing surface (ABS) of the write head and the pole piece layers are connected at a back gap. Current conducted to the coil layer induces a magnetic flux in the pole pieces which causes a magnetic field to fringe out at a write gap at the ABS for the purpose of writing the aforementioned magnetic impressions in tracks on the moving media, such as in circular tracks on the aforementioned rotating disk.
In current read head designs a spin valve sensor, also referred to as a giant magnetoresistive (GMR) sensor, has been employed for sensing magnetic fields from the rotating magnetic disk. The sensor includes a nonmagnetic conductive layer, hereinafter referred to as a spacer layer, sandwiched between first and second ferromagnetic layers, hereinafter referred to as a pinned layer and a free layer. First and second leads are connected to the spin valve sensor for conducting a sense current therethrough. The magnetization of the pinned layer is pinned perpendicular to the air bearing surface (ABS) and the magnetic moment of the free layer is located parallel to the ABS, but free to rotate in response to external magnetic fields. The magnetization of the pinned layer is typically pinned by exchange coupling with an antiferromagnetic layer.
The thickness of the spacer layer is chosen to be less than the mean free path of conduction electrons through the sensor. With this arrangement, a portion of the conduction electrons is scattered by the interfaces of the spacer layer with each of the pinned and free layers. When the magnetizations of the pinned and free layers are parallel with respect to one another, scattering is minimal and when the magnetizations of the pinned and free layer are antiparallel, scattering is maximized. Changes in scattering alter the resistance of the spin valve sensor in proportion to cos θ, where θ is the angle between the magnetizations of the pinned and free layers. In a read mode the resistance of the spin valve sensor changes proportionally to the magnitudes of the magnetic fields from the rotating disk. When a sense current is conducted through the spin valve sensor, resistance changes cause potential changes that are detected and processed as playback signals.
When a spin valve sensor employs a single pinned layer it is referred to as a simple spin valve. When a spin valve employs an antiparallel (AP) pinned layer it is referred to as an AP pinned spin valve. An AP spin valve includes first and second magnetic layers separated by a thin non-magnetic coupling layer such as Ru. The thickness of the spacer layer is chosen so as to antiparallel couple the magnetizations of the ferromagnetic layers of the pinned layer. A spin valve is also known as a top or bottom spin valve depending upon whether the pinning layer is at the top (formed after the free layer) or at the bottom (before the free layer).
The spin valve sensor is located between first and second nonmagnetic electrically insulating read gap layers and the first and second read gap layers are located between ferromagnetic first and second shield layers. In a merged magnetic head a single ferromagnetic layer functions as the second shield layer of the read head and as the first pole piece layer of the write head. In a piggyback head the second shield layer and the first pole piece layer are separate layers.
Magnetization of the pinned layer is usually fixed by exchange coupling one of the ferromagnetic layers (AP1) with a layer of antiferromagnetic material such as PtMn. While an antiferromagnetic (AFM) material such as PtMn does not in and of itself have a magnetization, when exchange coupled with a magnetic material, it can strongly pin the magnetization of the ferromagnetic layer.
The ever increasing demand for increased data rate and data capacity has lead a relentless push to develop magnetoresistive sensors having improved signal amplitude and reduced track width. Sensors that show promise in achieving higher signal amplitude include current perpendicular to plane (CPP) sensors. Such sensors conduct sense current from top to bottom, perpendicular to the planes of the sensor layers. Examples of CPP sensors include CPP GMR sensors and tunnel valve sensors. A CPP GMR sensor operates based on the spin dependent scattering of electrons through the sensor, similar to a more traditional CIP GMR sensor except that, as mentioned above, the sense current flows perpendicular to the plane of the layers. A tunnel valve operates based on the spin dependent tunneling of electrons through a non-magnetic, electrically insulating barrier layer.
Traditionally, the magnetization of the free layer has been accomplished by hard magnetic bias layers formed at either side of the sensor stack. However, the push for ever increased areal density has lead a continual push for sensors of ever decreasing size. As the size of the sensor shrinks, the free layer becomes less stable. The need for increased free layer stability is even more pronounced at higher signal amplitudes.
Another problem that arises as a result of the push for smaller sensors, is that as track width decreases, the magnetic performance of the sensor layers suffers. Manufacturing processes used to define the sensor (such as ion milling) cause a certain amount of damage to the sensor layers at the outside edges of the sensor. Areas nearer to the center are, however, unaffected by the manufacturing processes that damage the outside of the sensor. When sensors have a large track width, the damaged portion of the sensor layers at the outside of the sensor stack are a sufficiently small percentage of the sensor as a whole that the overall performance is not significantly affected. However, as sensor track width decreases, the damaged portion of the sensor makes up an unacceptably large percentage of the sensor, and performance suffers.
Therefore, there is a need for a sensor design that can provide increased free layer stability in CPP magnetoresistive sensor. There is also a need for a sensor design that can overcome the challenges presented by decreased track widths. Such a sensor would preferably overcome the performance problems caused by sensor damage at outside edges of the sensor stack.